Method of monitoring coalignment of a sighting or surveillance sensor suite

ABSTRACT

A method of monitoring the coalignment of a sighting or surveillance sensor suite including a coaligned laser (18) and sensor (12) includes the steps of: modifying the beam from the laser (18) to render it visible to the sensor (12); and redirecting the modified beam from the laser (18) to impinge on the sensor (12). In the preferred embodiments the frequency of the beam is doubled by a doubling crystal. For certain lasers this renders the beam visible to the human eye, or to a camera.

This invention relates to a method of monitoring the coalignment of asighting or surveillance sensor suite including a laser and a sensorcoaligned with the laser bean. The invention also relates to apparatusfor monitoring the coalignment of a sensor suite.

Modern military sighting and surveillance sensor suites are oftenrequired to have accurate coalignment of the sensors within the systemand, in such cases where weapons are to be aimed or guided, to the pointof impact of the weapon. Coalignment is achieved by one of severalmethods: the system may be factory set and coalignment retained bydesign; or in the case of a gun or rocket, the aiming device may be setby firing several practice rounds and adjusting the sighting systempoint of reference to the point of impact.

Maintaining alignment in a factory set system tends to result in overengineering of the aiming system to achieve the necessary long termstability, leading to cost and size/weight penalties. Also, anassumption that factory coalignment settings have been retained mayresult in problems and, in the case of a weapon system, the user isunable to determine how accurate his shot will be until he engages atarget. The impact on a surveillance system may not be as immediate, butrelying on inaccurate target location data could have seriousrepercussions.

Adjustment to sighting systems through monitoring the point of impact ofpractice rounds allows coalignment to be checked, though of course thisinvolves the deployment of ordinance. This requires provision of a safeclear area in which coalignment tests can be conducted, and may be timeconsuming, precluding use in theatre. Also, if the ordinance is costly,such as missiles or smart bombs, then such trials are economicallyunacceptable. Further, this form of trial requires the operator topossess a considerable degree of skill to adjust the system and providea subjective assessment of the error between the intended target and theactual point of impact of the projectile.

Increasingly, a greater number of weapons are laser guided, or havetargets illuminated by laser designators, and these systems dependheavily on high accuracy sensor coalignment. In such systems, the laseris the system reference and it is to the laser beam that the othersensors are coaligned.

One of the most popular lasers currently in use is the Nd: YAG laser.Lasers of this type are compact, solid state lasers emitting at 1064 nm.They are capable of producing good energy output (500 mj), at highrepetition rates (20 HZ and over), for typically, 15 ns pulse durations.However, in direct view sighting systems it is impossible to show theuser the path of the laser in order to effect coalignment because notonly is 1064 nm radiation invisible to the human eye, but can also causeserious eye damage.

Another difficulty in utilising laser based sighting or surveillancesensor suites is that, as mentioned above, the most popular lasers canpotentially cause serious eye damage. However, the requirement to trainmilitary personnel in the operation of laser based weapons systems in asnear real situation as possible requires use of such systems inexercises. To minimise the possibility of eye damage eye-safe lasershave been developed for training purposes. The most popular wavelengthof eye-safe laser operation is 1540 nm, as produced by erbium glasslasers. However, in a sighting system utilising CCD TV cameras it is notpossible to produce a coalignment checking system using 1540 nm energydirect onto the CCD as silicon, the basis for current CCD cameradetectors, does not absorb 1540 nm photons and therefore has no responseto this wavelength.

A number of techniques have been used to render lasers "visible" to suchsensors, and the human eye, the most popular of which relies on focusingthe laser onto a target formed of a material which absorbs the laserenergy and ablates to produce a visible spot. However, there are anumber of problems associated with such a system. Firstly, as the targetablates, it has a limited lifespan and ultimately it must be replaced,though its lifespan may be extended by employing a mechanical shiftingdevice to move the material and make maximum use of the target surface.Secondly, the visible laser spot tends not to be well defined. There area number of factors which contribute to this: the heating process causesan irregular plasma cloud to form above the material surface; the spotdefocusses as the surface is eroded; irregular ablation occurs becauseof faults in the material and features such as crystal grain lines; andthe ablation material reacts differently to each subsequent laser shotdue to residual effects of the previous shots.

One of these systems is disclosed in GB-A-2165957A for use with aimingapparatus including a laser and a thermal imager. Coalignment checkingapparatus contained within a housing is positioned in front of theaiming apparatus. The beam from the laser passes into the housing and isdirected to a concave mirror which focuses the laser energy on a bodywhich is then heated to give off thermal radiation. This thermalradiation is reflected and collimated by the concave mirror into a beamparallel to the laser sightline and within the field of view of thethermal imager. WO-A-87 06774 discloses a laser system for producing afrequency-doubled CW laser input beam. The system includes an Nd:YAGlaser and a KTP frequency-doubling crystal.

It is among the objects of the present invention to provide an improvedmethod and apparatus for use in monitoring the coalignment of a sightingor surveillance sensor suite including a laser and a sensor.

According to the present invention there is provided method ofmonitoring the coalignment of a sighting or surveillance sensor suiteincluding a laser and sensor which are coaligned such that the imagecreated by the beam from the laser impinging on an object is viewed bythe sensor, the method comprising the steps of: doubling the frequencyof the beam from the laser to produce a modified beam which is itselfdirectly visible to the sensor, and redirecting the modified beam toimpinge on the sensor.

The sensor may be an optical sensor or a CCD camera, or form part of adirect view sighting system. The laser may be one utilised for rangefinding, target designation and the like. In addition to use in militarysystems, the method may also be employed in laser ranging surveyingequipment and the like.

For use in a direct view sighting system utilising an Nd:YAG laser,frequency doubling renders the light visible to the human eye and, ifthe intensity of the modified laser is reduced, also renders the laserbeam nonharmful to the eye. Further, the resulting 532 nm wavelengthenergy is at the peak response of the eye. Thus, adjustment ofcoalignment is possible by directing the modified beam directly into thesighting system. For use in a CCD camera system utilising an erbiumglass laser, frequency doubling renders the modified beam visible whendirected into the camera and the resulting 770 nm radiation is,approximately, at the peak response of silicon-based CCD cameras. Thus,it may be seen that the present invention facilitates coalignmentchecking in a variety of laser based systems.

Preferably also, the method includes the further step of correcting thealignment of the laser beam and the sensor if the visible beam is foundto be out of alignment with the sensor: for example, the laser beam maybe moved using steerable optical elements; an aiming reference image maybe moved with respect to the outside world scene; or, in the case of acomputerised system, the alignment error may be entered into thecomputer for automatic compensation.

According to a further aspect of the present invention there is providedapparatus for monitoring the coalignment of a sighting or surveillancesuite including a laser and a sensor which are coaligned such that theimage created by the beam from the laser impinging on an object isviewed by the sensor, the apparatus comprising: means for redirectingthe beam from the laser to impinge on the sensor; and means for doublingthe frequency of the redirected beam to produce a modified beam which isitself directly visible to the sensor.

These and other aspects of the present invention will now be described,by way of example, with reference to the accompanying drawings, inwhich:

FIG. 1 is a schematic representation of a laser based missile sightingsystem incorporating apparatus for monitoring the coalignment of thesensor suite, in accordance with a preferred embodiment of the presentinvention;

FIG. 2 shows the normal aiming reference of the sighting system of FIG.1;

FIG. 3 shows the sighting system of FIG. 1, configured for determiningcoalignment; and

FIG. 4 is an enlarged view of the laser modifying device of the systemof FIGS. 1 and 3.

Reference is first made to FIG. 1 of the drawings which illustrates,somewhat schematically, a laser based missile sighting system 10. Thesystem includes an Nd: YAG laser 18, a beam splitter 19, two mirrors20,21 and a coalignment device 24, all located within the protectivecasing (not shown) around the sighting system. In use, the user,represented by eye 12, sees a small spot aiming reference 14 (FIG. 2)produced by the beam 16 from the laser 18 impinging on a target. Thespot is overlaid on the outside world scene which can be scanned usingthe steerable mirror 20. The returning part of the visible light createdby reflection of the beam 16 from the target is indicated by line 22.

To check the coalignment of the system 10, the mirror 20 is steered tothe position as illustrated in FIG. 3 of the drawings, such that theuser 12 is now viewing the coalignment device 24, as illustrated ingreater detail in FIG. 4 of the drawings.

The principle of operation of the device 24 will first be describedbriefly, followed by a more detailed description of various aspects ofthe device 24.

The laser energy 16a reflected by the mirror 20 enters the device 24 andis focused down into a specially processed zinc sulphide frequencydoubling crystal 28. Conveniently, the crystal 28 is formed of Cleartran(trade name), produced by Morton International. The crystal 28 doublesthe laser frequency and the 532 nm laser energy produced is reflectedback off a mirrored surface 30 on the back of the doubling crystal 28.The returned laser energy 16b passes back through the device 24 andenters the sighting system as an image of a spot, apparently atinfinity, or the point of focus of the sighting system. The image of thelaser energy spot is seen by the operator 12 as a green flash which canthen be aligned with respect to the cross-hairs 15 on the aimingreference (FIG. 2). This alignment can be accomplished in several ways:the input laser beam may be moved using steerable optical elements; theaiming reference image may be moved with respect to the outside world;or, in the case of a computerised system, the alignment error can beentered into the computer for automatic compensation.

The optics in the device 24 must be achromatic at the two wavelengths ofinterest, that is 1064 nm and 532 nm, in order to achieve good focus andalignment sensitivity. This is achieved in this embodiment through useof a doublet 32. The collection aperture of the device 24 is the fullaperture of the beam 16a and is an f5 optical system and the frequencydoubling crystal 28 is placed at the focal point of the incoming beam16a such that the mirrored rear surface 30 is at the focal point of thelaser. This ensures that the device is insensitive to tilt errors of thecrystal 28 and acts only as a retro-reflector, such that no errorsarising from manufacture of the device 24 are introduced into thecoalignment of the sighting system.

The mirror coating 30 on the doubling crystal 28 is a monochromaticreflector designed such that only the 532 nm wavelength is reflected.The unconverted 1064 nm energy passes through the filter and is absorbedin the laser dump 34 in which the crystal 28 is positioned.Conveniently, the surface of the dump 34 is painted with Nextel toabsorb any stray 1064 nm energy. As mentioned above, the preferredmaterial for the doubling crystal 28 is Cleartran, which is speciallyprocessed zinc sulphide. Ordinary zinc sulphide generates significantdispersion of the returned signal, which would result in an almostlambertian light output. This would lead to a very large, ill-definedreturn spot, as well as loss of return energy/energy density. It hasbeen found that the Cleartran crystal produces a well defined minimallyscattered 532 nm return pulse exactly coaligned with the original inputlaser beam but, because of the mirrored surface 30, in the oppositedirection. Beam vignetting is controlled by the alignment of the mirrorsurface tilt, but is not critical to successful operation.

The Cleartran crystal material also offers the advantage that itexhibits no polarisation sensitivity and has no critical thicknessrequirement; any polarisation state of laser energy can be input intothe device 24 and still give successful results, and the crystalthickness may be made suitable for handling and ease of production,without concern for the conversion process, though if the material istoo thin insufficient doubling occurs for the light to be visible.

The doubling process in the crystal 28 occurs when the electric fielddensity generated by the focused laser energy is of the order ofelectric field strength of the material, this typically representing asignificant laser energy density; approximately 10⁷ v/m is a typicalelectric field strength for most non-linear optical materials to beginto exhibit frequency doubling. The required energy density is less thanthe damage threshold of the Cleartran crystal 28, but any surfaceimperfections, particularly those at the mirror surface, at the focus ofthe laser, can result in lower damage thresholds.

A further consideration in the construction of the device 24 is theprotection of the user 12; the 532 nm energy is laser light and mirrorsexactly the input 1064 nm energy impulse duration. It is thereforenecessary to restrict the amount or converted energy reaching the eye ofthe user to safe limits. In this example, the restriction is effected byreducing the amount of the 1064 nm laser energy entering the device 24by using a KG5 glass plate 36 at the input to the device 24. At thislocation the light is in the form of a plane wavefront, such that theplate 36 does not affect the optical performance of the tool. As asecondary feature, any stray reflected 1064 nm energy will be attenuatedby the plate 36 as it leaves the device 24, thus protecting the userfrom stray unconverted energy.

Thus, this embodiment of the present invention provides a relativelysimple means of permitting coalignment of an Nd: YAG laser based directview sighting system. It will be clear to those of skill in the art thatthe invention may be used in other forms of sighting system, one ofwhich will now be described below.

In a CCD TV system a CCD camera 40 is provided at the image plane (inplace of the eye 12 illustrated in FIGS. 1 and 3) and the aimingreference is shown to the operator on a suitable viewing screen. For aCCD system for use with an erbium glass laser operating at 1540 nmcrystalline quartz is used as the doubling material. The wavelength(770) nm of the resulting laser energy is, approximately, the peakresponse wavelength of silicon CCD cameras which maximises systemsensitivity to the laser spot.

The operation of a quartz-based system is the same as the Cleartransystem described above, though the quartz is required to be morestringently dimensionally controlled and oriented with respect to thepolarization orientation of the input laser.

Quartz was selected as the frequency doubling material for thisapplication as it is readily and economically available, its parametersare well defined and it is insensitive to temperature change, animportant feature in this design. However, the quartz crystal needs tobe manufactured to very high optical standards of surface defect andimpurity inclusions to prevent the laser energy "picking-up" on thesesites and causing damage.

Further, the quartz component requires a tightly controlled thickness.To design a suitable frequency doubling target reference may be made toone of the relevant texts which will be familiar to those of skill inthe art, such as The Elements of Non-Linear optics (Chapter 7.2.1),edited by P N Butcher & D Cotter (Cambridge University Press, ISBN0-521-42424-0). However the governing equations are given below forreference: ##EQU1## Where I₂ω =irradiance of harmonic (Wm⁻²)

I.sub.ω =irradiance of fundamental (Wm⁻²)

K=constant

Z=crystal thickness

l_(c) =coherence length ##EQU2## Where c=velocity of light

ω=optical frequency

n.sub.ω =refractive index at fundamental frequency

n₂ω =refractive index at harmonic frequency ##EQU3## Where ε₀=permittivity of free space

d=second harmonic generation coefficient

For an angular error of θ in alignment, ##EQU4## Where I₂ω =intensityoutput for perfect alignment

In this case the 1540 nm energy is linearly polarised and thus usingpolarisation sensitive quartz requires that the crystal must becorrectly orientated to the input laser beam. After frequency doublingthe resultant 770 nm energy and 1540 nm energy have the samepolarisation state. Polarisation sensitive devices cannot, therefore, beused to separate them.

We claim:
 1. Apparatus for monitoring the coalignment of a sighting orsurveillance suite including a laser and a sensor which are coalignedsuch that the image created by the beam from the laser impinging on anobject is viewed by the sensor, the apparatus comprising: means forredirecting the beam from the laser; and means for (i) doubling thefrequency of the redirected beam to produce a modified beam which isitself directly visible to the sensor and for (ii) directing themodified beam back to the means for redirecting in order to impinge onthe sensor.
 2. The apparatus of claim 1, wherein the doubling means is adoubling crystal.
 3. The apparatus of claim 2, wherein the doublingcrystal is processed zinc sulphide.
 4. The apparatus of claim 2, whereinthe doubling crystal is crystalline quartz.
 5. The apparatus of claim 2,wherein the doubling crystal is provided with a mirrored rear surface.6. The apparatus of claim 5, wherein the mirrored rear surface of thedoubling crystal is located at the focal point of the incoming beam. 7.The apparatus of claim 6, wherein the mirrored surface is amonochromatic reflector and only reflects the modified laser beam. 8.The apparatus of claim 7, wherein a laser dump is located behind themirrored surface to absorb any unreflected energy.
 9. The apparatus ofclaim 1, for use in conjunction with a direct view sighting system,wherein the doubling means renders the laser beam visible to the humaneye.
 10. The apparatus of claim 1 including means for reducing theintensity of the laser beam.
 11. The apparatus of claim 1 in combinationwith a sighting suite in the form of a direct view sighting systemutilising an Nd:YAG laser, in which the beam from the laser is renderedvisible the eye.
 12. The apparatus of claim 1 in combination with asighting suite in the form of a CCD camera system utilising an erbiumglass laser, in which the beam from the laser is rendered visible to thecamera.